# CVE-2026-44040: UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes

> Attackers can predict VNC authentication challenges due to weak random number generation, potentially bypassing authentication or brute-forcing credentials.

- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 1 Jul 2026
- **Updated:** 9 Jul 2026
- **Also known as:** GHSA-j37g-g7hg-vhg6

## What is CVE-2026-44040?

CVE-2026-44040 is a medium-severity vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.

Weakness: CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG))

## Which versions are affected by CVE-2026-44040?

It affects 1 product.


Products: Uvnc Ultravnc.

## How do I fix CVE-2026-44040?

No fix has been published for CVE-2026-44040 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-44040 being exploited?

There is no known exploitation of CVE-2026-44040 in the wild and no public exploit code.

## Other vulnerabilities in uvnc UltraVNC

- [CVE-2019-8264](https://insights.sbomify.com/CVE-2019-8264/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8265](https://insights.sbomify.com/CVE-2019-8265/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8266](https://insights.sbomify.com/CVE-2019-8266/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8280](https://insights.sbomify.com/CVE-2019-8280/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2018-15361](https://insights.sbomify.com/CVE-2018-15361/): Memory corruption in UltraVNC (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-44040 and fix it if it is affected.

What it is: CVE-2026-44040 is a medium-severity vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-44040 in the wild and no public exploit code.

Affected product: uvnc UltraVNC. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-44040/ (Markdown: https://insights.sbomify.com/CVE-2026-44040/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-44040
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-44040
- OSV: https://osv.dev/vulnerability/CVE-2026-44040

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-44040/
Summaries are generated and may contain mistakes; the official description above is the record.